1. Field of the Invention
The present invention relates generally to clocks, and more particularly, to clocks for indicating the phase of the moon and clocks for indicating the state of the tide.
2. Background of the Invention
Clocks for indicating the phase of the moon (e.g., new, full) and clocks for indicating the state of the tide (e.g., low, high) have existed for years. Both "moon" clocks and "tide" clocks have utility in many applications.
Most moon clocks indicate the position and phase of the moon relative to the earth by providing a presentation of the lunar orbit. Typically, a disk depicting two diametrically opposed images of the moon is rotated behind the face of the clock at a rate of approximately one revolution every 59 days, i.e., one revolution every two lunar months. The 291/2 day lunar cycle is represented by one of the moon images as the disk rotates 180 degrees. The clock face shields appropriate portions of the representative moon image to provide an indication of the moon phase.
In many moon clocks, a graduated scale is disposed adjacent to the periphery of the disk. Each division of the scale represents a 24 hour day in the lunar cycle allowing the moon clock to be set to the nearest day and the number of days between moon phases to be measured.
Tides are primarily caused by the gravitational pull exerted on the earth by the moon and the sun. The gravitational pull exerted by the moon is the dominant force. Based on the 24 hour and 50 minute period of time in a lunar day, e.g., the average time between moonrise to moonrise on two consecutive nights, there is a high tide approximately every 12 hours and 25 minutes. A low tide typically occurs every 6 hours and 13 minutes after each high tide. Tide clocks are based on this time cycle. For example, most tide clocks indicate the occurance of a high tide every 12 hours and 25 minutes.
In fact, successive high tides and successive low tides do not always occur exactly 12 hours and 50 minutes apart. Due to many factors such as the relative locations of the moon and sun with respect to the earth and the inclinations of the orbits of the sun and moon with respect to the orbit of the earth, the tidal cycle varies throughout each month and throughout each year. As a result, although a tide clock can provide a rough indication of the time of the tides at certain times, it can be quite inaccurate at other times.
It has been discovered that the accuracy of a tide clock in indicating the times of the tides significantly varies depending on the time during the lunar cycle in which it is set. The times of certain tides on certain days of the lunar cycle correspond more accurately to the times that would be expected based on the average times of corresponding tides than do the times of other tides on other days. This discovery allows a tide clock to be set in such a manner that throughout the year the indicated times of the tides will show the least deviation from the actual times of the tides.
In order to set a tide clock in accordance with a particular time of the lunar cycle, the time of at least one moon phase must be determined. A tide clock can be set with reasonable accuracy based on the indication provided by an accurate moon clock.
By the present invention, an improved moon clock and an improved tide clock are provided. The indication provided by the moon clock is more precise than the indication provided by moon clocks developed heretofore. The tide clock provides a simplified presentation of the state of the tide. The precise indication provided by the moon clock can be used to set the tide clock in accordance with the discovery described above. A single clock comprising both the moon clock and the tide clock is also provided.